In a motor, when an insulation property reduces due to aged deterioration or the like of an insulating material, partial discharge occurs, whereby deterioration of the insulating material is promoted, eventually resulting in dielectric breakdown. It is required that partial discharge start voltage is higher than driving voltage for a motor. Therefore, in order to detect reduction in insulation performance at an early stage, it is necessary to perform insulation inspection by applying overvoltage equal to or higher than the driving voltage. A method and a device for performing partial discharge test for a motor, using an inverter for motor driving, without using a dedicated inspection device, are disclosed (for example, Patent Document 1).